They can try all they want. I see that occasionally on the websites people sell their stuff on. But usually even those people are willing to sell for a reasonable offer if their ad has been up for a long time.

I don't really see an issue with reselling niche items. The people looking for them aren't going to every thrift store weekly to find that exact item. They can conveniently go online and have it shipped to their door. It has caused goodwill to jack up prices and cherry pick all the good stuff for their own auction site.

I've bought and resold before as well as redold my old stuff on ebay but the margins can be thin unless you have a huge difference in price bought vs sold.

There's a big difference between resellimg and scalping.